Only option I've found is to replace, my O/S & N/S works as should, mirrors moves and the whole units fold in and out for parking, anyhow now and then the n/s will still move etc when unfolding but makes a loud clicking noise for a few seconds so I thought I would remove and have a look, this is when you find out that you cant strip the mirror down the motor is well protected and bonded into place, I've yet to figure out how to remove the bonding.
Why mention this, it's the same issue just with a slightly diff reason, overall its the motor parts become worn down making the fold option less tight than required / or allowing the wind to push back the mirror as the motor is failing to hold in place due to the same reasons - worn part.
You will notice on the bottom of the mirror is a circle, once removed you can see 4 small screws these hold the motor in place, dont bother to remove these. Take the mirror off the door and you will see the wire coming from the unit is bonded in. If you can get past this then you might have a chance at having a look at the motor/parts and seeing if you can tighten anything but I dought you can, it's all controlled by the motor.
Hand in pocket time or glue the housing up so it cant move....